Question

I think I’ve come across an interesting question – how to scramble, then undo the scramble on an array. Please keep in mind this needs to be done using .Net 2.0.

My thought process is shown below:

1. Somehow, sign an index of some sort to each item in the array
2. Randomize the array
3. Use the indexes to re-sort the array

byte[] b = new byte[] {1=>33,2=>19,3=>41,n=>N}
b.Sort();

The array might now look like this:

byte[] b = new byte[] {3=>41,2=>19,1=>33,n=>NN ... etc.};

Please, someone tell me what I’d like to do here is a possibility, and if it indeed is possible, please point me in the correct direction.

Thanks,

Evan

Answer 1

I would suggest shuffling and then unshuffling the array using the Fisher-Yates algorithm and a given integer key. The key can be fixed or random, but must be the same number for both operations in order to retrieve the original array.

Unscramble is a little bit more complicated than Scramble because the sequence of swaps must be generated using a Random initialised with the same seed, and then applied in reverse order.

void Scramble<T>(T[] array, int key)
{
    var random = new Random(key);
    for (int i = array.Length; i > 1; i--)
    {
        Swap(array, random.Next(i), i - 1);
    }
}

void Unscramble<T>(T[] array, int key)
{
    var random = new Random(key);
    var swaps = new List<int>(array.Length);
    for (int i = array.Length; i > 1; i--)
    {
       swaps.Add(random.Next(i));
    }
    swaps.Reverse();
    for (int i = 0 ; i < swaps.Count; ++i)
    {
        Swap(array, swaps[i], i + 1);
    }
}

void Swap<T>(T[] array, int i1, int i2)
{
    T tmp = array[i2];
    array[i2] = array[i1];
    array[i1] = tmp;
}